Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyzes metabolism of thiopurines. This metabolic pathway involving TPMT is critical for the metabolism of thiopurine-based drugs such as azathiopurine, 6-mercaptopurine, and 6-thioguanine. The level of TPMT activity varies considerably between individuals. Thus, identifying the level of TPMT activity that is present in an individual prior to thiopurine therapy can reduce or avert the onset of serious side effects.
TPMT activity exhibits genetic variation with about 89% of Caucasians and African-Americans having high TPMT activity, 11% with intermediate activity, and ˜1 in 300 exhibiting TPMT deficiency. The TPMT gene is 34 kb in size and consists of 10 exons.
The wild-type allele for high enzyme activity is designated TPMT1. Four variant alleles associated with decreased enzyme activity are designated TPMT3A, TPMT3B, TPMT3C, and TPMT2. Each variant allele harbors one or two mutations. The majority of the genetic variation in TPMT activity can be attributed to three single nucleotide mutations in the TPMT gene: 1) a guanine to cytosine substitution corresponding to position 238 of the DNA coding sequence (“G238C”) found in TPMT2, resulting in an amino acid change at position 80 of alanine to proline; 2) a guanine to adenine substitution corresponding to position 460 of the DNA coding sequence (“G460A”) found in TPMT3B, resulting in an amino acid change at position 154 of alanine to threonine; and 3) a adenine to guanine substitution corresponding to position 719 of the DNA coding sequence (“A719G”) found in TPMT3C, resulting in an amino acid change at position 240 of tyrosine to cysteine. The TPMT3A allele contains the G460A and the A719G mutations.
Methods for determining TPMT genotype have been described. See e.g., U.S. Pat. No. 5,856,095.